Plasma enhanced chemical vapor deposition of oxide film stack

ABSTRACT

A plasma enhanced chemical vapor deposition method is provided for depositing an oxide film onto a substrate surface. Deposition is achieved even onto a surface of a glass or other relatively non-receptive substrate. A sub-film is deposited under plasma enhanced chemical vapor deposition conditions more strongly favoring deposition, followed by deposition of the desired oxide film under second plasma enhanced chemical vapor deposition conditions less strongly favoring deposition. High quality oxide films can be achieved by deposition at second plasma enhanced chemical vapor deposition conditions only marginally favoring deposition over etching.

This application is a division of application Ser. No. 07/724,275, filedon Jul. 1, 1991.

INTRODUCTION

This application is directed to a plasma enhanced chemical vapordeposition method of depositing an oxide, for example, tungsten oxide orother infrared reflective refractory oxide. It is especially applicableto deposition of an oxide on the surface of a substrate which isrelatively non-receptive to such deposition.

BACKGROUND

An oxide film can be deposited on a substrate surface using a knownmethod generally referred to as plasma enhanced chemical vapordeposition ("PECVD"), also known as glow discharge deposition. Thisknown method is used commercially, for example, to deposit films ofphotovoltaic material on a substrate in the production of solar cells.PECVD involves ionizing a gas mixture which includes the species to bedeposited. For a film of tungsten oxide to serve as an electrode in anelectrochromic device, for example, the gas mixture, might comprisetungsten hexafluoride, oxygen and hydrogen gases.

Deposition of an oxide by PECVD involves a dynamic two-way process inwhich deposition and its opposite, surface etching, are occurringsimultaneously. Conditions must be controlled to favor deposition overetching. Specifically, parameters such as the composition of thedeposition atmosphere, the deposition temperature and ion bombardmentenergy, that is, the strength of any electrical field applied toaccelerate ions of the plasma toward the substrate surface to be coated,as well as secondary parameters such as flow rate and pressure, andpower level, all must be controlled to achieve deposition. Moreprecisely, these factors must be controlled to achieve net deposition,that is, to sufficiently favor deposition over etching in the dynamicprocess that the net result is an accumulating deposit of the desiredoxide.

Significantly, higher quality films, in particular, higher electronicquality films, are known to be achieved at PECVD deposition conditionsonly marginally favoring deposition over etching. Thus, for example, toachieve high quality films of crystalline tungsten oxide, the depositionconditions preferably are maintained close to those favoring etching.Unfortunately, however, oxide films often are not readily formed oncertain smooth surfaces at such marginal PECVD conditions. There is somespeculation that this deposition difficulty may be due to a lack ofnucleation sites on such smooth surfaces. In any event, there has beendifficulty in using PECVD techniques to form high quality oxide films oncertain substrates, such as glass and electronic quality dielectricsubstrates. While directed to deposition of a metal, tungsten, ratherthan an oxide, the difficulty of chemical vapor deposition ontodielectric surfaces, such as silicon dioxide, is shown generally in U.S.Pat. No. 4,777,061 to Wu et al. The Wu et al patent suggests an argonplasma pretreatment of the dielectric surface followed by low powerplasma deposition of tungsten, followed by thermal deposition oftungsten. That is, the Wu et al patent teaches that plasma deposition isterminated after deposition of an adhesion film, following whichdeposition proceeds by thermal decomposition. Plasma enhanced chemicalvapor deposition also is discussed in U.S. Pat. No. 4,572,841 toKaganowicz et al. An improved film having increased density is said tobe achieved in the Kaganowicz patent by introducing excess hydrogen intothe gaseous atmosphere during deposition. An optional first step is saidto involve oxidizing the surface of the silicon substrate by a plasmaoxidation method.

It is an object of the present invention to provide a PECVD depositionmethod to form oxide films on substrate surfaces. This and additionalobjects and features of the invention will be better understood from thedisclosure and discussion which follow.

SUMMARY OF THE INVENTION

In accordance with a first aspect of the invention, a plasma enhancedchemical vapor deposition method is provided for depositing an oxidefilm onto a surface of a substrate. The method comprises the steps ofdepositing a sub-film onto the surface at first PECVD conditions,followed by deposition of the oxide film over the sub-film at secondPECVD conditions. To achieve net deposition and substantially uniformfilm coverage, even over substrate surfaces which are relativelynon-receptive to deposition by plasma enhanced chemical vapor depositionmethods, the first PECVD conditions used in depositing the sub-film canstrongly favor deposition over etching. That is, one or more of thedeposition conditions, such as the composition of the depositionatmosphere, its pressure and flow rate, deposition temperature, ionbombardment energy and power level, are controlled to sufficiently favordeposition over etching to achieve net deposit of the sub-film. Thesecond PECVD conditions, however, while still favoring deposition overetching, less favor deposition than do the first PECVD conditions. Thus,the second PECVD conditions may only marginally favor deposition overetching, such that an oxide film of higher quality than the sub-film isdeposited. Nevertheless, net deposition of a substantially uniform andhigh quality oxide film can be achieved by plasma enhanced chemicalvapor deposition at marginal deposition conditions, notwithstanding thatthe substrate may be relatively non-receptive to PECVD deposition, afterfirst depositing the sub-film over the substrate. While not wishing tobe bound by theory, it presently is understood that net deposition ofthe oxide film over the entire surface is promoted by the sub-filmdeposited first onto the substrate surface at the aforesaid first PECVDconditions which more favor deposition over etching than do the secondPECVD conditions.

According to a second, particularly significant aspect of the invention,a glazing unit is provided comprising a substrate of glass, such assoda-lime glass typically used in architectural and automotive glazingapplications, and the oxide film is a refractory oxide, most preferablytungsten oxide, to provide infrared reflectivity. Such film may be used,for example, for solar load control, that is, to reduce the amount ofsolar energy in the form of infrared radiation passing through theglazing unit to an enclosed space. Rejection of such infrared solarenergy reduces the amount of air conditioning required to cool theenclosed space. According to this aspect of the invention, the abovedescribed differential plasma enhanced chemical vapor depositionconditions are employed for depositing a sub-film, for example, ofamorphous tungsten oxide, followed by a film of preferably crystallinetungsten oxide.

Several significant advantages may be realized in accordance withpreferred embodiments of the present invention. Most notably, plasmaenhanced chemical vapor deposition may be used to deposit oxide filmseven on surfaces which are relatively non-receptive to PECVD deposition.Moreover, the oxide films deposited in accordance with the invention maybe high quality films deposited under PECVD conditions only marginallyfavoring deposition, notwithstanding the non-receptive nature of thesubstrate surface. These and additional advantages and features of theinvention will be better understood from the following detaileddiscussion of certain preferred embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

Certain preferred embodiments of the invention are considered in thefollowing detailed description in conjunction with the accompanyingdrawing in which FIG. 1 is a cross sectional side view, partially brokenaway, illustrating an article comprising a substrate and an infraredreflective film stack on a surface of the substrate.

It should be understood that FIG. 1 is not drawn to scale. Inparticular, the films on the substrate surface are enlarged for clarityof illustration and ease of understanding.

DETAILED DESCRIPTION OF CERTAIN PREFERRED EMBODIMENTS

Plasma enhanced chemical vapor deposition of an oxide film on asubstrate surface is well known, generally, to those skilled in the art.Various oxide films are desirable due to their high conductivity, givingthem application as an electronic and/or ionic conductor inelectrochemical devices. In such applications, the oxide film willtypically be deposited on the surface of a tin oxide or othersemi-conductor substrate. Certain such oxide films also have excellentproperties for reflecting solar infrared radiation, giving themapplication as solar load control films for glazing units and the like.As noted above, however, plasma enhanced chemical vapor deposition is adynamic, two-way process in which both oxide deposition and surfaceetching (removing the deposited oxide) are occurring simultaneously. Itis well known to those skilled in the art that deposition conditionsmust be balanced to yield net deposition of the oxide. That is, adeposition rate greater than the rate of surface etching.

In accordance with certain highly preferred embodiments of theinvention, an oxide film, most preferably a tungsten oxide film, isdeposited onto a relatively non-receptive surface, for example, a glass,semi-conductor or other surface on which tungsten oxide normally doesnot accumulate well as a film under marginal PECVD conditions requiredfor high quality film formation, such as electronic quality filmformation or other crystalline tungsten oxide film formation. This isaccomplished according to such preferred embodiments by initiallydepositing a sub-film onto the substrate surface under first PECVDconditions which more strongly favor deposition over etching. Morespecifically, the first PECVD conditions sufficiently favor depositionof the oxide that the sub-film accumulates on the surfacenotwithstanding that the surface normally is relatively non-receptive toPECVD deposition of the oxide. The sub-film need only be thick enough torender the surface sufficiently receptive to deposition (i.e., net filmformation) of the desired higher quality tungsten oxide or other oxidefilm thereover. The sub-film preferably is about 100 to 1,000 Angstromsthick.

The material employed for the sub-film preferably is an oxide, such as ametal oxide, preferably tungsten oxide. According to certain highlypreferred embodiments of the invention, the sub-film is amorphoustungsten oxide. Since the sub-film is deposited under conditionsstrongly favoring deposition over etching, it can be expected that atungsten oxide sub-film in accordance with the invention will not becrystalline or electronic quality film. Rather, such tungsten oxidesub-film typically will be amorphous tungsten oxide. Most preferably foruse under a high quality, crystalline tungsten oxide film, an amorphoustungsten oxide sub-film is approximately 100 to 1,000 Angstroms thick,most preferably about 500 Angstroms thick. Alternative materials for thesub-film will be readily apparent to those skilled in the art in view ofthe present disclosure. Exemplary such alternative materials includesilicon carbide, silicon nitride, titanium dioxide, tin oxide anddiamond.

The thickness of the high quality oxide film deposited at second PECVDconditions over the sub-film will depend largely on the particularapplication for which the film is intended. It is within the ability ofthose skilled in the art to select appropriate film thicknesses forintended applications. In certain most preferred embodiments of theinvention, wherein an amorphous tungsten oxide sub-film is used under asubstantially transparent, infrared reflective, substantiallystoichiometric crystalline tungsten oxide top film is intended forautomotive or architectural glazing applications, the crystallinetungsten oxide top film preferably is about 500 to 10,000 Angstromsthick, more preferably about 2,000 to 4,000 Angstroms, most preferablyabout 2,000 Angstroms thick.

While tungsten oxide is preferred, other known refractory metal oxideswhich may be deposited by plasma enhanced chemical vapor deposition inaccordance with the present invention include, for example, titaniumdioxide and tin oxide. Suitable oxides in addition to the aforesaidmaterials will be readily apparent to those skilled in the art in viewof the present disclosure. Also, it will be within the ability of thoseskilled in the art to match suitable sub-film materials to the materialof the desired oxide film in accordance with the general principlesdisclosed and discussed herein. In addition, the oxide material,especially crystalline tungsten oxide, may be doped or oxygen deficientto effect its properties. Thus, in accordance with one preferredembodiment, a fluorine doped crystalline tungsten oxide film isdeposited in accordance with the invention over an amorphous tungstenoxide sub-film. The sub-film also may be fluorine doped or oxygendeficient. The tungsten oxide may be deposited slightly oxygen deficientor doped to increase its conductivity. In general, it will be apparentto those skilled in the art in view of the present disclosure, thatother modifications and variations of the deposited oxide sub-film andfilm may be employed in practicing consistent with the presentinvention. As used hereinafter the term oxide is intended to mean allsuch doped, oxygen deficient and/or otherwise modified oxide materials.Thus, in particular, the term tungsten oxide hereinafter is intended tomean tungsten oxide which may or may not be doped, for example, fluorinedoped, oxygen deficient, etc.

Referring now specifically to FIG. 1, a glazing article 10, such as amotor vehicle windshield or an architectural glazing unit, is seen tocomprise a planer substrate 12 preferably comprising glass, morepreferably soda-lime glass. Solar load control coating 16 has beendeposited by plasma enhanced chemical vapor deposition onto surface 14of glass substrate 12. Coating 16 includes a sub-film 18 of amorphoustungsten oxide directly on surface 14. Sub-film 18 has been depositedunder first PECVD conditions, discussed further below, which stronglyfavor deposition over etching. Film 20 of high quality, substantiallystoichiometric, substantially transparent, substantially crystallinetungsten oxide has been deposited under second PECVD conditions directlyonto sub-film 18. The second PECVD conditions only marginally favordeposition over etching, resulting in the high quality of film 20.Sub-film 18 is about 100 to 1,000 Angstroms thick. Film 20 is about2,000 to 4,000 Angstroms thick. A protective film 22 has been depositedby PECVD, sputtering or other suitable method directly on film 20 ofcrystalline tungsten oxide. In the preferred embodiment illustrated inFIG. 1, protective film 22, is silicon dioxide about 5,000 to 50,000Angstroms thick. Those skilled in the art, however, will recognize inview of this disclosure, that numerous alternative materials will besuitable for protective film 22 including, for example, silicon nitride,diamond, and aluminium oxide. The glazing unit of FIG. 1 has excellentsolar load control properties due, in part, to the infrared reflectivityof coating 16.

It will be within the ability of those skilled in the art, in view ofthe present disclosure, to select suitable PECVD conditions fordeposition of the film and sub-film of the invention. With particularreference to the preferred embodiment of FIG. 1, first PECVD conditionssuitable for deposition of the sub-film would be, for example, adeposition atmosphere consisting essentially of 25 sccm tungstenhexafluoride (WF₆), 25 sccm hydrogen (H₂), 50 sccm helium (He) and 50sccm oxygen (O₂) at a pressure of 25 mTorr, with applied microwave powerof 700 watts and substrate temperature of 125° C. These conditions willresult in deposition of an amorphous tungsten oxide film onto asubstrate such as soda-lime glass or a dielectric substrate. Depositionfor one-half minute typically will result in a sub-film about 500 to1,000 Angstroms thick, typically about 500 Angstroms thick.

A second set of PECVD deposition conditions suitable for depositing thehigher quality crystalline tungsten oxide film over the amorphoustungsten oxide sub-film would be, for example, a deposition atmosphereconsisting essentially of 25 sccm tungsten hexafluoride, 50 sccmhydrogen, 50 sccm helium, 50 sccm oxygen and 50 sccm carbontetrafluoride (CF₄) at a pressure of 40 mTorr, with applied microwavepower of 700 watts and a substrate temperature of 250° C. Depositionunder the aforesaid second PECVD conditions for a period of about oneminute results in a crystalline tungsten oxide film approximately 2,000to 6,000 Angstroms thick, typically about 3,000 to 4,000 Angstromsthick. Those skilled in the art will recognize that the aforesaid secondPECVD deposition conditions are marginal conditions in that they onlymarginally favor net deposition. That is, the second PECVD conditionsonly marginally favor deposition over surface etching. The film ofcrystalline tungsten oxide forms notwithstanding such marginaldeposition conditions in view of the receptivity of the surface of thesub-film. Such receptivity to deposition is in contrast to therelatively non-receptive surface of the soda-lime glass substrate ontowhich the sub-film is deposited. The resulting film of crystallinetungsten oxide is actually at least substantially crystalline. It alsois substantially transparent and substantially stoichiometric. Itsproperties include reflectivity of infrared radiation, notwithstandingthat it is substantially transparent to visible light. The film also issufficiently electrically conductive for use in certain electronicdevices. In addition, those skilled in the art will recognize that thefilm stack, in appropriate thicknesses, may advantageously be employedin certain electrical devices, for example, as an electrode in anelectrochromic device or the like, or as a resistance heating means fordeicing or defogging a windshield or other glazing unit carrying thefilm stack.

Those skilled in the art will recognize that PECVD deposition parametersor conditions are to a large extent interrelated. Thus, a change in afirst condition which would, by itself, result in deposition being morestrongly favored over etching, may be more than offset by a change inone or a combination of other deposition conditions. Bearing this inmind, the following general guidelines regarding PECVD deposition oftungsten oxide films and sub-films for the preferred embodiment of theinvention described above with reference to FIG. 1 are provided forelucidation and exemplification of the principles of the invention. Ingeneral, introduction of hydrogen gas into the deposition atmosphere, orincreasing the concentration of hydrogen gas in the depositionatmosphere, will produce a higher quality tungsten oxide film whilerendering the deposition conditions more marginal, that is, lessfavorable to deposition.

As in the preferred embodiment set forth above, the first PECVDconditions for deposition of the amorphous tungsten oxide sub-filmpreferably include a deposition atmosphere comprising no fluorine andless hydrogen, while the second PECVD conditions preferably include adeposition atmosphere comprising more hydrogen and 5 to 30 volumepercent of a fluorine-containing reactant. Increasing depositiontemperature, that is, the substrate temperature, also will produce ahigher quality tungsten oxide film while rendering the PECVD conditionsless favorable to deposition over etching. The PECVD conditions fordeposition of the sub-film preferably include a deposition temperaturein the range of room temperature to 200° C., while the second PECVDconditions for deposition of the higher quality crystalline tungstenoxide film thereover preferably include a deposition temperature ofabout 225° C. to 350° C. An additional condition which effects whetherthe conditions, as a whole, strongly or only marginally favor depositionover etching involves ion bombardment. Those skilled in the art arefamiliar with the use of an applied electric field to accelerate chargedspecies and induce ion bombardment of the surface as deposition isoccurring. In general, a stronger applied electric field results in moreenergetic ion bombardment and a higher quality film. However, etching or"sputtering" can occur at high ion energies. It will be within theability of those skilled in the art, aided by the present disclosure, toselect and control ion energy at suitable levels to yield strong ormarginal deposition conditions, taken in conjunction with the otherdeposition parameters. The pressure of the deposition atmosphere may belower during deposition of the sub-film to more strongly favordeposition over etching. Additionally, total flow rate and power levelmay be varied to effect deposition. Higher flow rate and higher powergenerally result in higher ion current density for ion bombardment.

Deposition methods and parameters for a protective film over thetungsten oxide film and sub-film, corresponding, for example, to film 22of FIG. 1, will be readily apparent to those skilled in the art in viewof the present disclosure. For deposition of a silicon dioxideprotective film, for example, suitable methods include sputtering,chemical vapor deposition and the like. Thus, for example, suitableconditions for plasma enhanced chemical vapor deposition would be 50sccm silane and 50 sccm carbon dioxide at a pressure of 25 mTorr, amicrowave power of 500 watts and a substrate temperature of 250° C.Deposition under the aforesaid conditions for a time period of about oneminute would result in a silicon dioxide protective film thickness ofabout 5,000 to 10,000 Angstroms.

It will be apparent to those skilled in the art that variousmodifications and variations can be made within the scope of the presentinvention. It is intended that all such obvious modifications andvariations which fall within the true scope of this invention beincluded within the terms of the appended claims.

We claim:
 1. An article comprising a substrate and an infraredreflective film stack on a surface of the substrate, the film stackcomprising a 10 to 100 nm thick sub-film of amorphous tungsten oxide anda 50 to 1,000 nm thick film of substantially stoichiometric, crystallinetungsten oxide on the sub-film.
 2. The article of claim 1 furthercomprising a 500 to 5,000 nm thick protective film of silicon dioxideover the film of crystalline tungsten oxide.